Voice over Internet Protocol, VoIP, is a general term for voice or speech communications and multimedia sessions over Internet Protocol, IP, networks. Other terms frequently encountered and synonymous with VoIP are IP telephony, Internet telephony, voice over broadband, VoBB, broadband telephony, and broadband phone.
VoIP systems employ session control protocols for the set-up and shut-down of communication sessions between a calling party, i.e. the party initiating the communication or call, and a called party, i.e. the party that is receiving the communication or call.
Connection related communication or signalling is generally handled in what is called a signalling domain or a control plane, separate from a content data exchange domain, generally referred as the user plane. The control plane is used by the calling and called parties to exchange control messages, for example messages to request, establish, and modify sessions. The exchange of media, for example digitized speech, audio, video, or data, between a calling party and a called party during a communication session occurs in the user plane.
Voice communication over a digitally operating IP communications network involves, among others, conversion of the analogue voice signal to a digital format and compression/translation of the signal into data packets for transmission over the IP communication network. Received data packets have to be expanded and converted back to an analogue voice signal. For producing such digital audio streams several codec's may be available, optimized for different transmission bandwidths, for example.
Users may avail themselves of a plurality of communication session related services, listed in a specific user profile, for example. Further aspects of a communication session comprise billing and provisioning of connections to communication networks external from the IP network. All or several of these aspects are processed in the control plane by different digital data servers of the IP communication network, such as subscriber registrars, also called subscriber registrar servers, Application Servers, AS, proxy servers, and the like.
The IP Multimedia Subsystem, IMS, telecommunications network is an architectural framework defined by the wireless standards body 3rd Generation Partnership Project, 3GPP, for delivering IP multimedia services to user terminals based on the Universal Mobile Telecommunications System, UMTS, network. A similar system called Multimedia Domain, MMD, was defined by the 3GPP2 for the CDMA2000 network, which was based on the 3GPP IMS. The IMS network architecture typically comprises a signalling plane or control plane and a content plane or user plane.
For network access, a user or party may connect to an IMS telecommunications network in various ways using Internet Protocols. IMS terminals, such as mobile phones, Personal Digital Assistants, PDAs, and computers, may register directly on an IMS telecommunications network, even when they are roaming in another network or country. Signalling messages in an IMS environment are exchanged using text based messaging, called Session Initiation Protocol, SIP.
Fixed access, e.g. Digital Subscriber Line, DSL, cable modems, Ethernet, mobile access, e.g. W-CDMA, CDMA2000, GSM, GPRS, and wireless access, e.g. WLAN, WiMAX, are all supported in IMS. Other phone systems like the Plain Old Telephone Service, POTS, or non IMS-compatible VoIP systems, are accessible in IMS through gateway servers.
For establishing a communication session in an IMS based VoIP network, first, in the control plane a signalling path is established between a calling party and a called party. The signalling path may involve various servers in the IMS network, for example SIP proxies, including a proxy server, such as Proxy-Call Session Control Function, P-CSCF, and subscriber registrar servers, also called Serving-Call Session Control Functions, S-CSCF, associated with both the calling party and the called party.
Typically, at least one AS is included in the signalling path for both the calling party as well as the called party, which is utilized for basic and supplementary telephone services, for example. An example of an AS server that is being developed in 3GPP is the Voice Call Continuity, VCC, Function server. Depending on the actual service, an AS can operate in a SIP proxy mode, SIP UA (User Agent) mode or SIP B2BUA (Back-to-Back User Agent) mode. Unlike a SIP proxy server, which only maintains transaction state, the B2BUA maintains complete communication session state and participates in all communication session requests. An AS server can be located in the home IMS network or in an external network, such as a third party service provider network.
In IMS an AS server is arranged for executing applications and services by manipulating SIP signalling and interfacing with other systems in the IMS telecommunications network. The AS server may also include HTTP capabilities allowing it to also perform the role of a content server for resources such as media files and VoiceXML application scripts. Typically, the AS server will offer a programming language and framework for creating new services, for example Java SIP and HTTP Servlets.
A chain of at least one subscriber registrar server and at least one application server, for both calling party and called party, is typical for an IP communication session in an IMS network.
During communication session establishment an important role is assigned to the subscriber registrar server. The subscriber registrar server applies, for example, a subscription check for invoking Value Added Services, VAS, to a communication session. Furthermore, the subscriber registrar server facilitates steps necessary for establishing the signalling path towards the called party.
To this end, the subscriber server in which the calling party is registered is arranged for, for example, applying number normalization and determining the destination network towards which the signalling path shall be established. This last step could involve domain name resolving, using Domain Name System, DNS, servers. The subscriber registrar server in which the called party is registered is arranged, among others, for registering contact address information to offer the communication session to one or more terminals of the called party.
When in the control plane a communication session is established, i.e. a session between the calling and called party is established, the active phase of the communication session involves transfer of media, such as audio, video, or data over a connection between the calling party and the called party in the user plane. Usually, the media in the user plane traverses fewer servers in the IMS network compared to the control plane. For example, in general, besides the calling terminal and the called terminal, only Media Proxies, MPs, are operative in the user plane.
Future utilization and the number of IP networks, such as IMS networks, are expected to grow gradually, due to, among others, the increasing demand for IP services of users with communication devices, such as mobile telephones. One of the challenges for operators of IP communication networks is to facilitate this increasing demand, and, at the same time, maintain or improve the reliability and robustness of the network.